Bear with me as I jump around wildly in this post, from Epsilon Eridani to happenings on our own Sun. The cause: Recent news about the solar wind from the Royal Astronomical Society’s meeting in Belfast that has me thinking about magnetic sails. The concept seems made to order for in-system propulsion. Instead of catching the momentum of solar photons with a large physical sail, try riding the flow of charged particles coming out of the Sun by using a magnetic sail generated aboard the vehicle. Velocities of several hundred kilometers per second seem feasible.
The thought of which reminded me to dig out a paper that Dana Andrews and Robert Zubrin presented at the 1990 Vision-21 symposium at NASA’s Lewis Research Center (now Glenn Research Center) in Cleveland. Andrews and Zubrin had written several papers on the concept, noting one way a magsail could operate. From the Vision-21 proceedings:
The magnetic sail, or Magsail, is a device which can be used to accelerate or decelerate a spacecraft by using a magnetic field to accelerate/deflect the plasma naturally found in the solar wind and interstellar medium. Its principle of operation is as follows: A loop of superconducting cable hundreds of kilometers in diameter is stored on a drum attached to a payload spacecraft. When the time comes for operation the cable is played out into space and a current is initiated in the loop. This current once initiated, will be maintained indefinitely in the superconductor without further power. The magnetic field created by the current will impart a hoop stress to the loop aiding the deployment and eventually forcing it to a rigid circular shape.
Other magsail concepts, like Robert Winglee’s M2P2 (Mini-Magnetospheric Plasma Propulsion) create a huge magnetic bubble around an interplanetary craft, an idea Winglee examined in two studies for NASA’s Institute for Advanced Concepts. But getting to the outer Solar System with magsails is one thing. Can we put the concept to work in interstellar missions? You wouldn’t think so, given the dispersion of the solar wind the further you move from the Sun, but Andrews and Zubrin realized that solar winds can be approached from two directions, one being the arrival of a starship at its destination, where braking becomes a critical function.
Two years before Vision-21, the two scientists had studied a potential one-way mission for a thousand ton payload to a star ten light years from Earth. Forget the magsail on the way out — the authors posited a lightsail pushed by a 1000 terawatt laser, initial acceleration limited by temperature constraints on the sail, and acceleration duration limited by the focusing capability of the laser optic system. The magsail would be deployed for deceleration into the target system, the total one-way trip time totaling 107 years.
But keeping interstellar journeys within a single human lifetime is obviously desirable. Fortunately, later work by Geoffrey Landis on dielectric sail materials allowed the authors to ramp up the acceleration. The 1990 paper posited a 5000 terawatt laser, which could reduce the travel time to 37 years when coupled with an improved magsail to reduce arrival times. Using a laser focusing mirror with a 50 kilometer aperture and a lightsail some 50 kilometers in diameter, Andrews and Zubrin figured 0.8 years for acceleration, 17.4 years coasting at roughly half the speed of light, and 18.8 years decelerating. The magsail is now 3100 kilometers in diameter versus 1000 in the earlier study, with deceleration times roughly half those found with the smaller sail.
Ten light years out gets you almost to Epsilon Eridani, assuming we find something there of interest. But let’s get back to our own Solar System. Before we can go magsailing even between planets, we need more information about how the solar wind operates. The work discussed at the Belfast meeting mentioned above pinpoints the source of the solar wind, using the UK-built Extreme Ultraviolet Imaging Spectrometer (EIS) aboard the Japanese Hinode spacecraft. The collision of magnetic fields from bright surface regions allows the requisite hot gases to flow out from the Sun. Says Louise Harra (UCL-Mullard Space Science Laboratory):
“It is fantastic to finally be able to pinpoint the source of the solar wind – it has been debated for many years and now we have the final piece of the jigsaw. In the future we want to be able to work out how the wind is transported through the solar system.”
Image: An X-ray image of the Sun made with the Hinode satellite on 20 February 2007. The insets show the flow of gas away from the bright region marked on the left. The blue image indicates material flowing towards us that will eventually make up the solar wind and the red image shows material flowing away from us back towards the surface of the Sun. Credit: L. Harra/JAXA/NASA/ESA.
This is the kind of jump we make when we study interstellar travel, from speculation about braking as we decelerate into another system to current research on our own star, work that is building the basis for what may one day become our first magsail deployments in space. Connecting the deeply speculative with the daily grind of ongoing research is what interstellar theorists are all about, the key being to keep the long-term goal in view even as we continue to build the necessary foundations.
The primary paper I’ve used for this discussion is Andrews and Zubrin, “Use of Magnetic Sails for Advanced Exploration Missions,” in the proceedings for Vision-21: Space Travel for the Next Millennium” (NASA Conference Publication 10059). Abstract here. The 1988 paper by the same authors is “Magnetic Sails and Interstellar Travel,” IAA Paper 88-553, presented at the 39th IAF Congress, Bangalore, India.
Hi Paul
One thing with magnetic sails is, in a uniform plasma medium, the sail accelerates highest at high plasma relative speeds. For an interstellar mission it means a lot of the decceleration happens very quickly and then it’s a slow crawl at low speed. The equation is ~ -3.k.u^4/3, where k is the magnetic sail constant and u is the relative speed between sail and medium. As you can see if the speed declines to 1/10 of its initial value, the acceleration declines to 1/21.5.
Makes me wonder, for quick deccelerations, if a magsail vehicle can’t be plunged in towards the target star, to deccelerate rapidly in the increased plasma density. The acceleration goes up with the 2/3 power of the relative plasma density. I’m yet to work out how fast a probe could deccelerate in an idealised solar-wind outflow, or how close to the star it needs to get for a given speed, but it’s interesting stuff.
Adam, here’s another thing to think about. Andrews and Zubrin also suggest carrying a fusion pulse engine to reduce the time spent in the ‘doldrums.’ What are the doldrums? This is interesting (from the 1990 paper):
“(The doldrums are the regions outside the boundary which separates the target star’s solar wind from the interstellar medium). The magsail is traveling between 3000 and 500 km/sec in this region and because the mass flow is so small, it takes several years elapsed time and almost a light year of distance to decelerate between these velocities. Adding a small rocket reduces the total deceleration period by four to five years.”
So we have plenty to ponder here in relation to how to approach a target system.
Hi Paul
Exactly what I was trying to avoid by a solar “fry-by”. Slowing from 0.5 c to 0.01 c takes 90% of the time to go from 0.01c to 0.000167c – Zubrin and Andrews are quite right about those doldrums. No big drama for a probe, but painful for a manned mission. A probe can do useful observations in the star’s Oort cloud, but people just want to ARRIVE!
Your mention of Robert Winglee’s M2P2 (Mini-Magnetospheric Plasma Propulsion) which creates a huge magnetic bubble around an interplanetary craft got me thinking about another impediment to space travel. The need for shielding from harmful rays and particles.
The Earth makes its own magnetic bubble which acts to shield us fairly well ,while the Ozone layer does the rest. If a ship using this propulsion can include the cabin inside the magnetic field properly, some of the shielding can be accomplished as part of the propulsion.
Thanks for a thoughtful article…
-Jeff Sheets
San Diego, CA
Pekka Janhunen from Finland indicates a potential problem with the M2P2 concept. Here’s what he says on the excellent website at:
http://www.hinduonnet.com/thehindu/fline/fl2408/stories/20070504006312300.htm
“When we analysed this, we found that a large fraction of the solar wind force will not be used in accelerating the spacecraft but in accelerating the plasma. It is the same physical phenomena as in a comet.”
If I understand what he is saying it’s that part of the plasma within the M2P2 will be lost and that what is lost will be traveling at fast enough speeds that a lot of momentum will not be transferred to the craft.
M2P2 is such a great idea I’d hate to see us loose it. But what about the criticism? I have not seen any response to it.
Hi Paul and Adam;
The more I think of 10 EXP 15 Watt class lasers, the more I think they could be designed and assembled in Solar orbit in a crash program should we detect signs of life, even electromagnetic transceiving noise from an extrasolar planet.
The U.S. Air Force’s soon to be operational 747 ABL platform that will use a COIL laser with a beam output of roughly 10 MW to destroy ICBMs in flight will include the elements Chlourine, Oxygen, and Iodine to produce the killer beam which will last about 3 seconds per shot for many shots. These lasers are a whopping 70 to 80 percent effecient.
I can imagine a whole bank of COIL laser modules, perhaps a billion of them providing a beam of 10 EXP 16 watts, but wherein, obviously, there would be a need to provide the cooling of the modules to prevent over heating. The chemical reactants could be recycled by a thermodynamic gradient derived from concentrated solar energy. Huge inflatable or otherwise deployable membranous high mass specific power output reflectors could be used to collect the necessary sunlight to operate the system.
Assuming that ultimately 1/10 of the beam energy is converted into ship based KE over an continuous operating cycle of 5 years, (10 EXP 16)(3 x 10 EXP 6)(5) Joules = 1.5 x 10 EXP 23 Joules of KE could be imparted to a ship. If the ship and sail had a total mass of 1,000 metric tons, the ship would reach about 0.92 C with a gamma factor of 3, thus putting stars within a 50 lightyear radius reachable in one human familiar generation. If 1/5 of the beam energy could be converted into ship based KE, then a gamma factor of about 6 could be reached thus putting stars within a 90 to 100 lightyear radius reachable by one contemporary human family generation ship’s reference frame.
Construction would still be a huge undertaking and would obviously cost in the 10s of trillions of dollars just to produce the COIL modules at a reduced mass produced cost of 10s of thousands of dolllar each. But, given some bold futuristic UN like mandate, it just might be pulled off sometime this century.
Thanks;
Jim
The mag-sail method would certainly be a boon to interplanetary exploration. For manned missions, not only would the magnetic field be used for flight, a ‘combo’ type could also be used to protect the crews from cosmic rays; http://www.space.com/businesstechnology/technology/technovel_magnetic_041217.html
Remarkable how this topic logically follows to the discussion in the other post about Dyson spheres, Oort cloud comets and interstellar travel.
@James, thanks, your calculations are a good elaboration of what I was trying to say in that other discussion: that such a laser (10^15 W = 1000 TW) would be a big undertaking, but probably feasible for humankind in the foreseeable future.
Suppose we really detect habitable planets near Alpha Centauri and make it a priority: 1% of global GDP would be about 60 billion US$ per year, which over 50 years would amount to some 30,000 billion or 30 trillion US$ (net present value, i.e. not correcetd for inflation).
There you have your ’10s of trillions of dollars’, James, spread over 50 years.
Hi Ronald;
Thanks for the enthusiatic response to my previous posting. We as an international community need to start thinking really big. An international based manned interstellar mission program would utterly unite the human race. I could see the major automakers producing 100 million COIL modules per year using all of their manufacturing wizardry. What a great program for China to mass produce the coil modules also.
Thanks;
Jim
James: thanks for your enthousiastic and interesting ideas and forward thinking.
Correction: ‘1% of global GDP would be about 60 billion US$ per year’, that should have been 600 billion US$ per year. The total over 50 years remains.
It seems like a lot per year, but the US presently spents some 800 billion US$ on defense, directly and indirectly, such as ‘global war on terror’ and Irak (which is not part of the regular defense budget, but an appropriation).
I find it a sad idea, in relation to the above, that for a couple of billion, we could have had SIM, TFP, and some ground-based ELT’s, already.
For some tens of billions per year, we could have had the beginning of colonization on Mars and a continuous human presence on the moon.
We (I mean the western world) spend more even on cosmetics or snacks.
Hi Folks;
It seems as if the only real absolute limit of travel velocity thru space for an beamed electromagnetic energy powered craft is C itself.
If a collimnated beam could be produced having a power output of the Sun, a space craft traveling so close to C such that the dopplar losses attenuating the beam energy to target by a factor of 4 x 10 EXP 10 would still permit the collection of 10 EXP 16 Joules/sec by the craft at this level of dopplar loss Earth’s reference time frame. In reality, the craft could be accellerated to a gamma factor of significantly in excess of one billion wherein the optical power of the beam would start out small, say around 10 EXP 16 Watts wherein after billions of years Earth time of ship based 1 G accelleration or about 55 years ship time accelleration over 13.73 billion LYs in spatial distance, the craft would reach a point at which its recessional velocity from Earth would exceed C as a result of the space time expansion of the intervening spacetime.
A laser beam with a final power output of say 10 EXP 30 Watts, and an initial power output of say 10 EXP 18 watts would permit accelleration of several Gs and much higher gamma factors by the time the craft reached the observable universe limit.
Note that the formula for relativistic dopplar redshift is z = {[ 1 + (v/c)]/{[1 – [(v exp 2)/(c exp 2)]] EXP 1/2}} – 1. So when v ~ c, z = {[1 + 1](gamma)} -1 ~ 2(gamma) for high gamma factors. As a result, even a craft traveling at a gamma factor of 5 x 10 EXP 11 would still receive and effective 10 EXP 18 watts from a beam with originating power of 10 EXP 30 watts. For a 1,000 metric tons rest mass craft, that would still yield many Gs of accelleration.
The craft having left the horizon of the observable universe with respect to Earth, would in theory continue to have its recessional velocity with respect to Earth and the Milky Way mount to ever higher superluminal velocities.
Regards;
Jim
Hi James
Nice idea there, though I expect some breakthroughs with lasers in a few years as the full range of applications of nanotubes comes into play.
Hi Adam;
Thanks for the response.
I am looking forward to any nanotube revolution whether it is in strength of materials, near superconductivity or superconductivity, lasers, or whatever. The age of the nanotube will no doubt have good use in manned interstellar travel.
Regards;
Jim
From 2001 don’t exist any advancing in M2P2 or magsail studies. Why is a such bad results?
AlfaCentavra, I don’t have a recent update from Robert Winglee re M2P2, but in general magsail and for that matter solar sail studies are simply facing budgetary problems as NASA looks at funding other programs of a more short-term nature. Needless to say, the lack of fundamental research (and the recent closing of NIAC, not to mention the earlier loss of the Breakthrough Propulsion Physics program) has dismayed many of us who push for human expansion into the cosmos. ESA and JAXA have all done interesting solar sail work in the past, as has NASA, and it’s to be hoped that future budgets are more supportive. All we can do is keep making the case, and also investigate philanthropic funding alternatives for research, about which more in the near future in these pages.
A magnetic sail can double as a laser light sail, if it is loaded with an ion species with a strong resonance absorption line (singly ionized alkali earth metals, for example). The cross section for scattering at resonance can be immense, and this would largely eliminate sail heating concerns, since if the laser frequency is chosen appropriately it can actually cool the plasma (by selectively scattering off plasma ions with velocity in the spacecraft reference frame toward the light source, causing them to on average lose kinetic energy in that frame).